This study discusses some problematic issues around comparability and equivalence in empirical research, particularly focusing on the case of indicators and taking a cross-national study on linguistic skills and use of foreign languages in the scientific production of researchers from Argentina, Brazil, and Chile as reference. The discussion is framed within broader epistemological and methodological debates concerning the status of comparisons in the social sciences and, mainly, the case of cross-national comparative studies. Within these debates, and resorting to concrete examples arising from research into scientists’ linguistics skills, we address the comparability issue of target populations and samples, as well as that of indicator equivalence, with emphasis in the problems of questionnaire design, reliability, and validity.